The present invention relates to a brazing filler for sealing vacuum-tight vessels, vacuum-tight vessels and a method for manufacturing vacuum-tight vessels.
The vacuum-tight vessels used in vacuum valves, thyristor etc. are manufactured by sealing the open end of a ceramic cylinder with a sealing metal cap to maintain vacuum-tight conditions inside the vessels.
Conventionally, such vacuum-tight vessels have been manufactured by placing a Mo--Mn metallized layer between the open end of a ceramic cylinder and a metal sealing cap and then sealing the cylinder with the sealing cap by heating. However, this method is problematic in that the step of producing the Mo--Mn metallized layer is complicated and thermal treatment at high temperature is required for sealing.
Under these circumstances, an active metal method has been investigated as a sealing method as a substitute for the method using the Mo--Mn metallized layer. A brazing filler consisting of active metal is generally a filler containing 3 elements of Ag, Cu, and Ti. Japanese Patent Publication No. 1216/1960 discloses that ceramic and metal are joined via a brazing filler prepared by adding Ti to an eutectic composition of Ag--Cu. However, if such brazing filler consisting of active metal is placed between the open end of the ceramic cylinder and the metal sealing cap and heated at the melting point of the brazing filler, the whole of the brazing filler is melted all at once. As a result, the brazing material cannot demonstrate sufficient wetting properties towards the cylinder and thus it was difficult to seal the open end of the cylinder with the metal sealing cap.